The present invention generally relates to liquid crystal displays and battery tester circuits, and more particularly to battery tester circuits of the type that may be printed on a battery label.
Battery tester circuits exist that may be printed on a battery label. Such existing battery tester circuits are typically either "thermochromic" testers or "electrochromic" testers. Thermochromic testers include a calibrated resistor that is selectively coupled to the opposite poles of the battery through a switch that may be provided at either or both ends of the calibrated resistor. A thermochromic ink is printed over the resistor that responds to changes in temperature of the calibrated resistor to gradually change between opaque and transparent states and thereby enable indicia printed under the thermochromic layer to be viewed or blocked based upon the temperature of the calibrated resistor. Alternatively, the thermochromic layer may change colors in response to the temperature of the calibrated resistor. The temperature of the calibrated resistor is determined by the power which the battery can deliver, which is a function of both the voltage and internal resistance of the battery. The accuracy of a thermochromic tester is determined by not only the rate of change of the open circuit voltage and internal resistance (rate of change of the battery's ability to produce power), but also the sharpness of the color change in the thermochromic ink (the number of degrees of temperature change required to make the thermochromic ink change color). Thus, the thermochromic ink layer functions both as a display and temperature sensor.
Electrochromic testers differ from thermochromic testers in that the display layer changes color directly in response to the "open circuit voltage" of the battery. The accuracy of an electrochromic tester is determined by the rate of change of the open circuit voltage of the battery with depth of discharge and the sharpness of the change of intensity of the electrochromic display with voltage. Thus, like the thermochromic tester, the electrochromic tester display functions both as a display and a voltage sensor and the accuracy of the tester may be limited by the voltage response of the display.
Since the accuracy of these thermochromic and electrochromic testers is limited by the response of the display, it has been proposed to improve tester accuracy by including a voltage-responsive electronic component, such as a Zener diode or transistor and to thus limit the function of the display to that of a display. Such an approach is disclosed in U.S. Pat. Nos. 5,610,511, 5,460,902, and 5,389,470. In these patents, a tester circuit is disclosed that utilizes discrete electronic components to discriminate between various discharge levels and to selectively activate different segments of a thermochromic display. Thus, these tester circuits provide discrete displays for the various discharge levels that may be discriminated by the separate sensing circuit thereby limiting the function of the display to that of a display. However, because the testers disclosed in these patents utilize discrete electronic components manufactured using conventional semiconductor technology, the electronic components are not small enough to be included in the label of a battery. Further, because the exterior dimensions of batteries are strictly limited by the ANSI standards, such electronic components cannot be provided on the exterior surface of the battery. If such electronic components were to be provided in the interior of the battery, the space occupied by the electronic components would reduce the space in which the active battery ingredients are provided thereby reducing the service life of the battery. For these reasons, the use of a separate voltage discrimination circuit for an on-label tester has not been commercially implemented.
Another problem associated with thermochromic and electrochromic testers concerns the amount of power consumed by these testers. Because these testers consume relatively significant levels of power, switches are provided to enable selective activation of the testers without requiring a constant drain on the battery. Because of the requirement for such switches, however, the displays do not continuously display the current discharge level of the battery.
Although general purpose electric field-responsive liquid crystal displays are known, they are too expensive to include on a battery label and they require activation voltage levels well in excess of the open circuit voltage of most batteries. Further, these liquid crystal displays tend to irreversibly polarize when driven using a direct current (DC) driving signal. For these reasons, field-responsive liquid crystal displays have been considered to be unsuitable for use in an on-label battery tester.